A carbonic ester is a monomer useful in preparation of polycarbonates, and many studies have been made on preparation of the carbonic ester. Conventionally, a carbonic ester has been prepared through reaction of an alcohol with phosgene as a carbonyl source. However, this method has a problem of use of phosgene, which is a very toxic compound, and also brings problems with use of a chlorinated solvent and disposal of neutral by-product salts.
To overcome these problems, a method for preparing a carbonic ester using carbon monoxide as a carbonyl source has been developed. However, when carbon monoxide is used as a carbonyl source, there are problems of reduction in reaction speed and yield, a high risk of explosion due to use of toxic carbon monoxide at high pressure, and costs for securing safety. In addition, there is a concern of side reactions, such as generation of carbon dioxide caused by oxidation of carbon monoxide.
There has also been developed a method wherein carbon dioxide is reacted with ethylene oxide and the like to prepare a cyclic carbonic ester, followed by reacting the cyclic carbonic ester with methanol, producing dimethyl carbonate. This method has advantages in that harmless carbon dioxide is used as a carbonyl source, and corrosive materials are hardly generated. However, this method can cause side reactions such as generation of ethylene glycol, and has a limitation relating to plant location due to difficulty in safe transport of ethylene oxide or ethylene, which is a source of ethylene oxide.
Recently, a method for preparing a carbonic ester by reacting carbon dioxide with an organometallic compound has been studied. It has also been found that, after separation of a carbonic ester from a mixture produced by the method, the organometallic compound can be reproduced by reaction of a residual liquid with an alcohol. In other words, the used organometallic compound can be recycled to be reused in formation of the carbonic ester, thereby eliminating transport-related problems. As such an organometallic compound, there are disclosed compounds in the form of Sn(R)2(OR′)2 (R and R′ being two different alkyl groups) which includes tin as a center metal and contains two alkyl groups and an alkoxy group (Japanese Patent Publication No. 2010-523783 A, No. 2006-548937 A, No. 2006-513613 A, No. 2007-269653 A, No. 2005-511122 A, No. 2003-556375 A, and the like). However, these known organometallic compounds have a problem of low reactivity and reduction in production yield of a carbonic ester. Therefore, there is a need for a method which can overcome these problems.